Sea lice (Copepoda, Caligidae) are the most widely distributed marine pathogens in salmon industry in the last 30 years. They also spread in the final 15 years to other culture species and wild salmonid populations (Pike, A. W. y Wadsworth, S. L. (2000). Advances in Parasitology 44:233-337, Ragias, V. et al. (2004). Aquaculture 242: 727-733). There are three major genera of sea lice: Pseudocaligus, Caligus and Lepeophtheirus. 
Considering salmonid production throughout northern hemisphere, one of these species, Lepeophtheirus salmonis, is the responsible for the main disease outbreaks in salmonid farms. This parasite, just in 2004, was the responsible for direct and indirect losses in worldwide aquaculture, of 100 millions of US dollars (Johnson, S. C., et al. (2004). Zool Studies 43: 8-19). All the sea lice developmental stages in which the parasite is attached to the host, feed on host mucus, skin and blood. The sea lice adhesion and feeding produce lesions that differ in their nature and severity, depending on the sea lice specie, their abundance, the developmental stages present and the host specie (Johnson, S. C et al., “Interactions between sea lice and their hosts”. En: Host-Parasite Interactions. Editors: G. Wiegertjes and G. Flik, Garland Science/Bios Science Publications, 2004, pp. 131-160). In the southern hemisphere, Caligus rogercresseyi is the most important specie affecting chilean salmon industry (González, L. y Carvajal, J. (2003). Aquaculture 220: 101-117). In the case of severe illness, like the one observed in Atlantic salmon (Salmo salar), when the fish are infected by a high number of L. salmonis, extensive damaged and hemorrhagic skin areas are observed in the head and fish back. Also, a distinct area of erosion and sub-epidermal hemorrhage in the perianal region can be seen (Grimnes, A. et al. (1996). J Fish Biol 48: 1179-1194). Sea lice can cause host physiological changes including a stress response, reduction of immune functions, osmorregulation failure and death, if the infection is not treated (Johnson, S. C., et al. (2004). Zool Studies 43: 8-19).
A wide range of chemicals had been used to control sea lice infestations like hydrogen peroxide, organophosphates, ivermectin and other related compounds like emamectin benzoate, molting regulators and pyrethrins (MacKinnon, B. M. (1997). World Aquaculture 28: 5-10; Stone J., et al. (1999). J Fish Dis 22: 261-270). The treatments against sea lice can be apply by immersion baths like organophosphates and pyrethroyds or orally, as ivermectin. These immersion baths are difficult to perform. In addition they are expensive and can have significant effects over fish growth after treatments (MacKinnon, B. M. (1997). World Aquaculture 28: 5-10). Besides, the chemicals commonly used by immersion baths are not effective in all sea lice developmental stages. To date, the use of oral treatments such as SLICE® (emamectin benzoate) is predominant in salmon industry. SLICE®, unlike chemicals administered by immersion, give a short protection against re-infection. This treatment, although is easier to apply compared to immersion baths, is also expensive and requires a period of time before the fish can be destined for human consumption (Stone J., et al. (1999). J Fish Dis 22: 261-270).
There are evidences which suggest the development of resistance against conventional treatments in L salmonis, particularly in populations frequently treated (Denholm, I. (2002). Pest Manag Sci 58: 528-536). This fact together with the necessity of reducing costs and threats to the environment, make imperative the development of new approaches like vaccines to control sea lice infestations in fish. The experience with terrestrial parasites demonstrated that a successful vaccine needs to be comprised for one or more concealed antigens with low or not homology with the host proteins. Sea lice are ectoparasites that feed on host mucus, skin and blood and then, only have limited contact with the host immune system (Boxaspen, K. (2006). ICES Journal of Marine Science 63: 1304-1316). In these cases, it was observed host immune response suppression due to the production of immunomodulatory proteins by the parasite at the adhesion and feeding site (Wikel, S. K., et al., “Arthropod modulation of host immune responses”. En: The Immunology of Host-Ectoparasitic Arthropod Relationships. Editors: Wikel, S. K., CAB Int., 1996, pp. 107-130). These proteins have been investigated for their use as candidate vaccines to control sea lice infestations. They have been patented and have been evaluated in assays performed in vitro to study its effects over the host immune system (Patent No. WO2006010265: RECOMBINANT VACCINES AGAINST CALIGID COPEPODS (SEA LICE) AND ANTIGEN SEQUENCES THEREOF). Tripsins, vitellogenin-like proteins and host adhesion proteins are some of the molecules studied as potential antigens (Johnson, S. C., et al. (2004). Zool Studies 43: 8-19; Boxaspen, K. (2006). ICES Journal of Marine Science 63: 1304-1316).
In general, vaccines are safer than chemical treatments for both, fish and environment. Nevertheless, until now there are not commercial vaccines available against sea lice. Experimental vaccines against L salmonis, which employ animal whole extracts, have been developed. These vaccines were not protective since their administration resulted in minor changes in L salmonis fecundity (Grayson T. H., et al. (1995). J Fish Biol 47: 85-94).
The identification of targets for sea lice prevention and treatment have not been successful as a result of the poor knowledge about the mechanisms involved in the pathology of sea lice infestations in salmons. This handicap makes difficult the progress of researches related to recombinant vaccines development. The reality is that up to the moment an effective vaccine against these ectoparasites has not been developed.
In other arthropods like ticks, genes involved in different tick's genera reproduction and feeding (Almazán et al. (2003). Vaccine 21:1492-1501) have been identified, employing expression immunization libraries. The results based on RNA interference (de La Fuente et al. (2005). Parasitol Res. 96:137-141) and immunization trials (Almazán et al. (2005). Vaccine 23: 4403-4416) suggest that these genes might be good candidates for vaccine development against different tick species which infest mammals. One of the tick's studied proteins, cement protein, which is produced in the tick salivary glands appear to be a good candidate to confer protection against different tick species (Adama, R. et al. (2005). Vaccine 23: 4329-4341) and consequently, against opportunistic pathogens which use ticks as their hosts (Labuda, M. et al. (2006). PLoS Pathogens 2(4): 251-259).